It is cultivated in China for food, but was introduced in Europe and the United States for aquatic weed control. It is a fish of large, turbid rivers and associated floodplain lakes, with a wide degree of temperature tolerance. Grass carp will enter reproductive condition and spawn at temperatures of 20 to 30 °C (68 to 86 °F).[2][3]

In the United States, the fish is also known as white amur, which is derived from the Amur River, where the species is probably native, but has never been abundant.[2] This is not to be confused with the white Amur bream (Parabramis pekinensis), which is not a particularly close relative.

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Appearance and anatomy

Grass carp have elongated, chubby, torpedo-shaped body forms. The terminal mouth is slightly oblique with non-fleshy, firm lips, and no barbels.[4] The complete lateral line contains 40 to 42 scales. Broad, ridged, pharyngeal teeth are arranged in a 2, 4-4, 2 formula. The dorsal fin has eight to 10 soft rays, and the anal fin is set closer to the tail than most cyprinids. Body color is dark olive, shading to brownish-yellow on the sides, with a white belly and large, slightly outlined scales.

The grass carp grows very rapidly. Young fish stocked in the spring at 20 cm (7.9 in) will reach over 45 cm (18 in) by fall. The average length is about 60–100 cm (23.5–39.5 in). The maximum length is 2.0 m (6.6 ft) and they grow 100 kg (220 lb). The grass carp's maximum weight is 99 lbs. According to one study, they live an average of five to 9 years, with the oldest surviving 11 years. In Silver Lake Washington there is a thriving population of grass carp passing the 15 year mark.[5] They eat up to three times their own body weight daily. They thrive in small lakes and backwaters that provide an abundant supply of freshwater vegetation.[citation needed]

Ecology

This species occurs in lakes, ponds, pools, and backwaters of large rivers, preferring large, slow-flowing or standing water bodies with vegetation.[4] In the wild, grass carp spawn in fast-moving rivers, and their eggs, which are slightly heavier than water, develop while drifting downstream, kept in suspension by turbulence. The eggs are thought to die if they sink to the bottom.[6]

Adults of the species feed primarily on aquatic plants. They feed on higher aquatic plants and submerged terrestrial vegetation, but may also take detritus, insects, and other invertebrates.[2][7]

Introduced species

Grass carp have been introduced to many countries around the world. In the Northern Hemisphere, countries and territories of introduction include Taiwan, Israel, Japan, the Philippines, the United States, Mexico, India, Malaysia, the Netherlands, Switzerland, Czech Republic, Slovakia, Croatia, Serbia, Slovenia, Montenegro, Bosnia and Herzegovina, Macedonia, Denmark, Sweden, Romania, Poland, Italy, Germany, France, and the United Kingdom.[citation needed] In the Southern Hemisphere, they have been introduced to Argentina, Venezuela, Fiji, New Zealand, Australia, and South Africa. Grass carp are known to have spawned and established self-reproducing populations in only six of the many larger Northern Hemisphere rivers into which they have been stocked. Their failure to establish populations in other rivers suggests they have quite specific reproductive requirements.[8]

In the United States, the species was first imported in 1963 from Taiwan and Malaysia to aquaculture facilities in Alabama and Arkansas.[9] The first release is believed to have been an accidental escape in 1966 from the US Fish and Wildlife Service's Fish Farming Experimental Station in Stuttgart, Arkansas, followed by planned introductions beginning in 1969.[9][10] Subsequently, there have been widespread authorized, illegal, and accidental introductions; by the 1970s the species had been introduced to 40 states, and it has since been reported in 45 of the country’s 50 states.[9][10] In 2013 it was determined to be reproducing in the Great Lakes Basin.[11] It is still stocked in many states as an effective biocontrol for undesirable aquatic vegetation,[9][10] many species of which are themselves introduced.

Grass carp require long rivers for the survival of the eggs and very young fish.

Use as weed control

Healthy, live, and swimming grass carp as seen from above

Grass carp were introduced into New Zealand in 1966 to control the growth of aquatic plants. Unlike the other introduced fish brought to New Zealand, the potential value and impact of grass carp was investigated in secure facilities prior to their use in field trials.[12] They are now approved by the New Zealand government for aquatic weed control, although each instance requires specific authorization.[13] In the Netherlands the species was also introduced, in 1973, to control over-abundant aquatic weeds. The release was controlled and regulated by the Dutch Ministry of Agriculture, Nature and Food Quality.

In both of these countries, control is made easier by the fact that grass carp are very unlikely to naturally reproduce because of their very specific breeding requirements,[13] but elsewhere control is obtained by the use of sterile, triploid fish.[10][14]

Fishing for grass carp

A grass carp caught using white bread and six-pound test, monofilament line

Grass carp grow large and are strong fighters on a rod and reel, but because of their vegetarian habits and their wariness, they can be difficult to catch.[15]Chumming with corn adds to success. They will eat canned corn, cherry tomatoes, and, despite their primarily herbivorous habits, will also sometimes eat other animals. Chumming with white bread, and a piece of bread pinched on a hook and floated on the surface works well, especially for pond grass carp. The IGFA World record for a grass carp caught on line and hook is 39.75 kg, (87.6 lb) caught in Bulgaria in 2009.[16]

Where grass carp populations are maintained through stocking as a biocontrol for noxious weeds, fishers are typically asked to return any caught to the water alive and unharmed.

The fish are popular among bowfishers where bowfishing for them is legal.